Oil and natural gas produced from oil fields and gas fields contain associated gas. The associated gas contains corrosive gas, such as carbon dioxide gas (CO2) and/or hydrogen sulfide (H2S). Line pipes transport oil and natural gas containing the above corrosive gas. Consequently, in line pipes, stress corrosion cracking (SCC), sulfide stress cracking (SSC), and general corrosion cracking account for reduction in wall thickness may cause problems in some cases.
SCC and SSC cause rapid propagation of the cracking. Hence, SCC and SSC penetrate line pipes in a short time since they occur. In addition, SCC and SSC occur locally. For theses reasons, corrosion resistance, particularly, SCC resistance and SSC resistance are required in steel material for use in line pipes.
Duplex stainless steel has high corrosion resistance. Hence, duplex stainless steel is used as steel for line pipes.
High strengthening of steel pipes attains reduction in wall thickness of the steel pipes for the line pipes, resulting in reduction in production cost. In this sense, high strengthening is required in the duplex stainless steel for use in the line pipes. JP 2003-171743A (Patent Literature 1) and JP 5-132741A (Patent Literature 2) suggest duplex stainless steel having high strength.
Patent Literature 1 discloses the following: the duplex stainless steel of Patent Literature 1 contains Mo of at least 2.00% as well as W. Solid-solution strengthening of Mo and W enhances strength of the duplex stainless steel. The duplex stainless steel of Patent Literature 1 contains Cr of 22.00 to 28.00%, and Ni of 3.00 to 5.00%. This configuration enhances corrosion resistance of the duplex stainless steel.
Patent Literature 2 discloses the following: the duplex stainless steel of Patent Literature 2 contains Mo of at least 2.00% as well as W. In the duplex stainless steel, PREW=Cr+3.3 (Mo+0.5 W)+16N is at least 40. The contents of Mo and W enhance strength of the duplex stainless steel. PREW of at least 40 enhances corrosion resistance of the duplex stainless, as well.